1. Field of the Invention
The present invention relates to a defect inspection method and a defect inspection apparatus for inspecting defects in an object to be inspected, such as a silicon wafer.
2. Description of the Related Art
Crystal defects of a silicon wafer or the like include oxidation induced stacking faults (OSFs) and bulk microdefects (BMDs). When a silicon wafer is etched, etch pits of a few xcexcm depth may appear in the surface thereof. Etch pits exhibiting regularity corresponding to a face-centered tetragonal structure of a silicon wafer are referred to as the OSFs, while etch pits having irregular shapes are referred to as BMDs of oxidation deposits, microdisplacement, and stacking faults. Usually, the inspection for the OSFs or BMDs is visually performed with the aid of a microscope. The defects are visually checked and the number of such defects in an inspection visual field is counted to implement quality assurance and quality control of wafers on the basis of the counting result.
There has also been proposed methods whereby to capture microscopic images and process the images so as to detect and count defects. Examples of such methods are found in Japanese Unexamined Patent Application Publication No. 61-194737 entitled xe2x80x9cSILICON WAFER OSF DENSITY INSPECTION METHODxe2x80x9d and Japanese Examined Patent Application Publication No. 6-71038 entitled xe2x80x9cCRYSTAL DEFECT RECOGNIZING METHOD.xe2x80x9d
Here, according to a first defect inspection method, potential defects that have been processed into binarized images on the basis of the shape characteristics of defects are checked to determine whether they are truly defects. For instance, according to the method disclosed in Japanese Unexamined Patent Application Publication No. 61-194737, potential defects are checked against a reference shape to determine whether the candidates are truly defects. The method according to Japanese Examined Patent Application Publication No. 6-71038 involves the length and aspect ratio of a defect. These inspection methods focus only on the shapes of defects to make decisions. More specifically, only a reference shape or the shape of a defect involving the length or aspect ratio of the defect are used as determining parameters to decide whether the defect is a true defect.
According to a second defect inspection method, a predetermined threshold value is set for a captured image to binarize the image thereby to detect potential defects and to determine whether they are true defects on the basis of the characteristic amounts of the potential defects. Then, the quantity of the true defects is counted.
The first inspection method, however, presents the following problem since it uses only the shape of a potential defect as the determining parameter.
In the case of the potential defects shown in FIG. 2, judging from the shapes thereof, they may be determined to be OSFs, while it still remains uncertain from the aspect of their distribution of occurrence. More specifically, of the seven potential defects found in the visual field, six are very closely located. Such potential defects are normally produced from accidental contact of tweezers or some other foreign matters to the surface of a wafer during a process of etching a silicon wafer, and therefore, are not true defects that occurred on the surface of the wafer.
The conventional inspection method, however, makes it impossible to accurately discriminate the potential defects caused by tweezers coming in contact with the surface of a wafer. As a result, it is determined that there is a greater number of defects than there really is, thus interfering with accurate quality control.
The second inspection method presents the following problem.
The number of defects in an area measuring a few hundreds of xcexcm squares per visual field is counted, meaning that the inspection of the whole surface of a wafer involves a few hundreds or tens of thousands of visual fields to be inspected. In the case of visual inspection, at least about one second is required to inspect one visual field, so that inspecting all visual fields takes too much time. Furthermore, human factors, such as the degree of fatigue of an inspector, inevitably lead to inconsistency in inspection results.
Although the method in which microscopic images are processed to count the number of potential defects solves the problem in the above visual inspection, the method presents the problem described below. Since captured images are simply processed into binarized images to detect defects, if there are many defects in an inspection visual field, as shown in FIG. 3A, it may happen that a plurality of closely located defects candidates shown in FIG. 3B are overlapped and misjudged to be a single defect. Especially in the case of BMDs, numerous defects frequently take place, so that the defects may be overlapped and detected as a single defect, thus posing a problem in achieving reliable quality assurance and quality control.
In addition, if a standard microscope is used, a captured defect of a few xcexcm depth may be overlooked due to poor contrast, presenting another problem in achieving reliable quality assurance and quality control.
Accordingly, the present invention has been made with a view toward solving the problems discussed above, and it is an object of the present invention to provide a defect inspection method and a defect inspection apparatus that improve determination accuracy by using the distribution of detected defects as a parameter for determining whether a potential defect is a true defect, and also improve defect detection accuracy by preventing human errors and by accurately identifying micro-defects and closely located defects.
To this end, according to one aspect of the present invention, there is provided a defect inspection method for inspecting potential defects observed on a surface of an object to be inspected, wherein the number and the positions of the detected potential defects are used as determining parameters for determining whether the potential defects are true defects.
With this arrangement, the number of detected potential defects and the locations of the detected potential defects are used as the parameters for determining whether the potential defects are true defects, making it easier to recognize the potential defects (not true defects) that occurred due to contact of a foreign object, including tweezers, with a surface of an object to be inspected, such as a silicon wafer. The contact by a foreign object, such as tweezers, causes potential defects to take place closely located. More specifically, such potential defects may densely occur around a point or along a line. Therefore, it is possible to determine whether detected potential defects are true defects by measuring the density of the potential defects based on the number of detected potential defects and also the positions thereof.
Preferably, the density of potential defects is determined on the basis of the aforesaid number of potential defects and the positions thereof, and the determined density is compared with a preset value so as to decide whether the potential defects are true defects.
With this arrangement, if the determined density of potential defects based on the number of potential defects and the positions thereof exceeds the preset value, then it is determined that the potential defects are not true defects.
According to another aspect of the present invention, there is provided a defect inspection method wherein, to inspect potential defects observed on a surface of an object to be inspected, if a total number of potential defects in an inspection visual field is smaller than a preset value, then an area that is smaller than the inspection visual field is set, the gravity center position of a detected potential defect is aligned with the central position of the foregoing area, the number of potential defects in the area is counted, the density of the potential defects is determined on the basis of the ratio of the counted number of potential defects to the total number of potential defects in the inspection visual field, and it is decided whether the determined density of the potential defects exceeds a preset percentage so as to determine whether the potential defects are true defects.
With the arrangement discussed above, it can be determined that the density is higher as there are more potential defects in the area set to be smaller than the inspection visual field. On the other hand, the defects caused by the contact of a foreign object, such as tweezers, to a surface of an object to be inspected also lead to a higher density. For this reason, if statistical numerical data on the densities of defects resulting from generally assumed foreign objects is obtained by sampling or the like in advance, and the percentages against the whole inspection visual field are preset, then comparison with the preset percentages makes it possible to easily determine whether potential defects are true defects. In other words, if the densities are higher than the preset percentages, then it is determined that the potential defects are not true defects.
According to still another aspect of the present invention, there is provided a defect inspection method wherein, to inspect potential defects observed on a surface of an object to be inspected, if a total number of potential defects in an inspection visual field is smaller than a preset value, then the gravity center positions of all potential defects are determined, an n-order approximate curve is determined on the basis of the distribution of the gravity center positions, the number of potential defects existing in a set width along n-order approximate curve is counted, the density of the potential defects is determined on the basis of the ratio of the counted number of the potential defects to the total number of potential defects in the inspection visual field, and it is determined whether the density of the potential defects is a preset ratio or more so as to determine whether the potential defects are true defects.
In the configuration described above, a foreign matter, such as tweezers, may point-contact or line-contact with a surface of an object to be inspected, such as a silicon wafer. In the case of the line contact, the distribution of the gravity center positions of potential defects will be linear. Hence, an n-order approximate curve is drawn along the distributed gravity center positions of the potential defects. If the density of potential defects along the n-order approximate curve is high, then it is determined that the potential defects are not true defects.
According to yet another aspect of the present invention, there is provided a defect inspection method wherein, to inspect potential defects observed on a surface of an object to be inspected, if a total number of potential defects in an inspection visual field is smaller than a preset value, then the gravity center positions of all potential defects are determined, and the distances among the potential defects are calculated on the basis of the gravity center positions to determine the mean value of the distances. Then, it is determined whether the distance that is smaller than the value obtained by adding or subtracting an offset value preset for the determined mean value is a preset ratio or more against the number of combinations of the distances among all potential defects in the inspection visual field so as to determine whether the potential defects are true defects.
This arrangement makes it possible to determine the density of potential defects and to determine whether the potential defects are true defects on the basis of the determined density, as in the case of the aspects of the present invention discussed above.
In a preferred form of the present invention, if it is determined that observed potential defects are not true defects, then inspection is carried out in a visual field in the vicinity of the inspected visual field.
With the arrangement discussed above, in the case of potential defects caused by contact of a foreign object or the like, then the visual field adjacent to an inspected visual field is inspected to determine the condition of a true defect. In this case, a concentric adjoining visual field is most likely to be inspected.
In a preferred form of the present invention, the inspection results at two positions that are symmetrical on the whole surface of an inspected object are compared, and it is determined whether the difference therebetween exceeds a preset value so as to determine whether potential defects are true defects.
In the above arrangement, at the two symmetrical positions, the numbers of true defects are considered to be substantially the same, due to the CZ method. According to the CZ method, an ingot is drawn up while rotating it. At this time, crystal growth simultaneously occurs on a concentric circle, usually producing the same state. Hence, if defects occur, then the defects tend to be concentrically distributed. This means that whether potential defects are true defects can be determined by comparing the states of potential defects at a plurality of concentric spots. More specifically, the inspection results at two symmetrical positions are compared, and if the difference therebetween exceeds a preset value, then the potential defects are considered to be attributable to some external cause and it can be determined that they are not true defects.
According to a further aspect of the present invention, there is provided a defect inspection method wherein a surface of an object to be inspected is captured using a differential interference microscope, the obtained image is processed to count the number of potential defects observed on the surface, and potential defects are detected at the spots where luminance is different in the captured image.
With the arrangement described above, an image captured using the differential interference microscope shows defective spots exhibiting irregularities compared with the rest of a surface, and the luminance therefore is different at the defective spots in the captured image. Thus, it may be determined that the spot having a difference luminance has a potential defect, and the spot is checked for a true defect.
In a preferred form of the present invention, a spatial filter is applied to the captured image to enhance the spot exhibiting the different luminance, and the enhanced spot is subjected to a binarizing process to detect a potential defect. Based on the characteristic amount of the detected spot, it is determined whether the potential defects are true defects or noises, then the number of true defects is counted.
This arrangement enables potential defects to be reliably recognized by applying the spatial filter to enhance spots where luminance changes. Moreover, a plurality of adjoining potential defects can be also clearly identified as separate individual potential defects. Binarizing the spots that have been enhanced by the spatial filter clarifies potential defects, permitting easier detection. Then, based on the characteristic amounts of the detected spots, it is determined whether the potential defects are true defects or noises, thus allowing the number of only true defects to be counted.
According to a further aspect of the present invention, there is provided an inspection apparatus adapted to capture a surface of an object to be inspected by a differential interference microscope and to carry out image processing to count the number of potential defects observed on the surface, the inspection apparatus being equipped with a defect detecting unit for detecting potential defects at spots exhibiting different luminance in the captured image.
With the arrangement, the differential interference microscope displays the irregularities of captured defective spots in terms of different luminance, and the defect detecting unit detects potential defects at the spots having the different luminances in the captured image.
Preferably, the defect inspection apparatus is equipped with a boundary enhancing unit that applies a spatial filter to the captured image to enhance a boundary area where luminance changes, and processes the boundary area into a binary-coded image to obtain a clear potential defect image, a defect detecting unit for detecting the potential defects that have been enhanced and clarified by the boundary enhancing unit, and a defect counting unit that determines whether the potential defects are true defects or noises on the basis of the characteristic amount of the area detected by the defect detecting unit, then counts the number of true defects.
With this arrangement, the boundary enhancing unit enhances a boundary area by using a spatial filter to clarify the presence of potential defects. Even if a plurality of adjoining potential defects is present, each of the potential defects can be clearly identified and recognized. The defect detecting unit binarizes the area enhanced by the spatial filter of the boundary enhancing unit so as to clarify and detect potential defects. Then, based on the characteristic amount of the detected area, the defect counting unit determines whether the detected potential defects are true defects or noises, and counts the number of only true defects.